BP, in a series of newspaper advertisements about the Deepwater Horizon disaster, says it is “working around the clock to contain and collect most of the leak” and it will “take full responsibility for cleaning up the spill.”

But if past catastrophes are guides, the cleanup by BP workers will capture only a fraction of the crude belched up by the broken well. Much of the oil will be taken care of by nature; the rest is likely to stay with us for decades.

In Alaskan coastal zones fouled by the Exxon Valdez in 1989, scientists discovered oil, scarcely changed, 16 years later. In some areas, its composition had not altered much from the toxic clumps and goo that had formed just weeks after the spill.

Contrary to early expectations, oil still oozes from Alaska’s beaches, toxins intact, and is expected to remain — perhaps even for centuries.

The BP disaster differs greatly from the Valdez. In the Gulf of Mexico, vast plumes of oil, attacked with harsh chemical dispersants, churn up from mile-deep waters. In Alaska, a surface slick swept over more than a thousand miles of rocky coast. Spilled oil behaves in many ways, but here, based on sad experience, is some of what to expect in the gulf.

EVAPORATION Some oil on the water surface evaporates within days. The lighter the oil, the more evaporation: half or more of light crudes can evaporate; medium crudes, up to 40 percent evaporate; heavy crudes as low as 10 percent. The spilled oil in the gulf is light crude, according to Edward B. Overton, an environmental sciences professor at Louisiana State University; Exxon Valdez oil was heavier.

DISPERSAL Oil may be reduced to tiny droplets by wave action or chemicals. Droplets are more readily consumed by microbes, but the effects of toxic chemical dispersants used in the gulf are not known.

EATEN BY BACTERIA If conditions are right, microbes can consume a great deal of spilled oil. But this is not always possible in oxygen-starved environments, like the deep sea or where oil has seeped into beach sediments.

REMOVAL Armies of workers at the Exxon Valdez disaster — 11,000 at the effort's peak — removed less than half the oil that didn't evaporate or biodegrade. Here is what happened to the more than 40,000 tons of Valdez oil, according to a study conducted three years after the spill.

What Remains

OIL ‘MOUSSE’ Residual emulsified oil — a gooey mix of crude from the Exxon disaster and water, remains in beaches in Alaska.

BURIED OIL Crude that penetrated into coastal sediments remains in dismaying amounts in Alaska. When sand and rock is disturbed — by burrowing or foraging animals, or surf — oil may leach out. Otherwise the oil remains intact and resistant to degradation. Pictured above: oil from a small hole dug on May 5 on Eleanor Island, Alaska.

TAR BALLS AND ASPHALT Congealed oil forms tar balls that resist weathering and can last for years; mix sand or beach gravel with oil and you get asphalt, which also resists erosion. Tar balls washed up on Florida beaches in the early days of the gulf spill, but they were not from Deepwater Horizon — they may have formed from other spills, or occurred naturally from oil seeps in the ocean. Pictured above: a tar ball in Pensacola, Fla. on Friday.

INTO THE FOOD CHAIN Oil and toxins concentrate in filtering animals like mussels, oysters and clams and are then ingested by their predators. The long-term effects of this are not fully understood, but oil ingestion is known to damage animals’ immune systems and organs and cause behavioral changes that affect the ability to find food or avoid predators.

SLOWED RATES OF OIL LOSS Most of the oil from the 1989 Valdez disaster disappeared in the first few years. Scientists had hoped that almost all the remaining oil would disappear soon after, based on how quickly the oil was degrading in the early 1990s.

But later surveys showed that this oil was much slower to degrade, leading scientists to fear that it may persist for decades. About 100 tons of oil was still in the beaches of Prince William Sound as of 2001, out of more than 20,000 tons deposited there. And it is easily uncovered there today.

A version of this article appears in print on June 6, 2010, on Page WK3 of the New York edition with the headline: Even With a Cleanup, Spilled Oil Stays With Us. Order Reprints|Today's Paper|Subscribe